I noticed a "Declined" suggestion, but think I have the same issue. Found an article on the various ERG modes in the TPV Help Centre. This does not offer the ERG mode required.

Terrain Mode. When in terrain mode, the resistance you feel will be the resistance provided by the terrain you can see – hills, descents, wind etc.
Fixed Mode. When in fixed mode, the resistance you feel will be fixed – equivalent to riding on an infinitely flat road on a windless day.
ERG Mode. When in ERG mode, the resistance you feel will be continually updated to match the current wattage target in your workout – this means that you will always hit the indicated target power (if your legs can supply it!) without needing to concentrate on your virtual surroundings or exactly what effort to make.

The requirement is that during workout performed in ERG mode, you are able to "switch off" ERG and default back to normal gearing when you are no longer able to push the required watts. The terrain should remain neutral as to not allow elevation changes to interfere with the workout.

This is similar to "Fixed Mode", but it allows rider to default back to normal gearing controlled from the bike. It removes the "fixed resistance" at possibly a level that is no longer achievable (hence trying to "switch off" ERG).